(1) Field of the Invention
The present invention relates to an aluminum nitride sintered body and a process for the preparation thereof. More particularly, the present invention relates to an aluminum nitride sintered body having a high heat conductivity, which is suitable as an electronic part material for a radiating substrate or the like, and a process for the preparation thereof.
(2) Description of the Related Art
With recent increase of the performance and processing speed in information-processing devices, increase of the density and integration degree has been promptly advanced in semiconductor integration circuits constituting these information-processing devices. Accordingly, a large electric power becomes necessary for a semiconductor integrated circuit element and the quantity of heat generated in the element is drastically increased. Therefore, in order to operate the semiconductor integrated circuit element normally and stably, it is very important to remove generated heat efficiently.
In a conventional semiconductor package comprising alumina as a substrate, the heat conductivity is low and the radiation of heat is insufficient. Therefore, a beryllium oxide sintered body has been proposed as a ceramic material having a high heat conductivity. However, use of this sintered body is restricted because of its toxicity.
An aluminum nitride sintered body now attracts attention as a highly heat-conductive substrate material that can be used instead of the beryllium oxide sintered body, because this sintered body has a high mechanical strength within a broad temperature range of from normal temperature to high temperatures, a high electrically insulating property, a high heat conductivity and a thermal expansion coefficient closer to that of a single crystal of silicon than alumina.
However, aluminum nitride inherently has a hardly sintering property, and when aluminum nitride alone is used, it is difficult to obtain a high-density sintered body having a high heat conductivity. Accordingly, a sintered body has been obtained by incorporating, as a sintering aid, a compound of an element of group IIa or IIIa of the Periodic Table, for example, a compound of an alkaline earth metal such as calcium, strontium or barium, or yttrium or a rare earth element, to the starting material powder of aluminum nitride.
For example, U.S. Pat. No. 4,478,785 proposes a process in which a composition formed by adding carbon to aluminum nitride powder is molded and sintered to remove oxygen (contained in the form of aluminum oxide) contained in the starting aluminum nitride and improve the heat conductivity of the sintered body. However, this proposal is still insufficient in that the action of the sintering aid is not satisfactory, the density of the sintered body is low and the heat conductivity is rather degraded.
U.S. Pat. No. 4,547,471 discloses a process in which a composition formed by adding Y.sub.2 O.sub.3 as a sintering aid to aluminum nitride powder is molded and sintered. According to this process, a sintered body having a high density can be obtained, but the heat conductivity of the sintered body is still insufficient. The reason is that the sintering aid reacts with an oxide phase contained in aluminum nitride to form a grain boundary phase and promote sintering of aluminum nitride but the sintering aid is still left in the grain boundary phase even after termination of sintering. Accordingly, color unevenness and stain are observed in the sintered body and the heat conductivity is locally uneven. Therefore, the process is defective in that a sintered body having a good heat conductivity cannot be prepared easily and stably.
U.S. Pat. No. 4,746,637 discloses a process in which a composition formed by incorporating a compound of a metal of group IIa of the Periodic Table, such as calcium, strontium or barium and a compound of a metal of group IIIa of the Periodic Table, such as yttrium or other rare earth element, in a starting powder of aluminum nitride is molded and sintered. Also in this process, the sintering aid reacts with the oxide phase on the surface of aluminum nitride and is left as the grain boundary layer, and the above-mentioned defect is similarly caused.
A process in which the amount of the sintering aid is reduced as much as possible to decrease the grain boundary phase can be considered as means for eliminating the above-mentioned defect. However, since the sintering aid is an element indispensable for sintering aluminum nitride, if the amount of the sintering aid is too small, a sintered body having a high density cannot be obtained.